1. Field of the Invention
This invention is concerned with certain novel carbazole-based molecular photoconductive charge-transfer complexes, and more particularly to methods for synthesizing these carbazole-based molecular photoconductive charge-transfer complexes and their application to electrosubstrates, as well as methods for constructing photoconductive elements on thin semitransparent flexible substrates.
2. DESCRIPTION OF THE PRIOR ART
Carbazole-nased materials such as polycarbazoles are well known in the prior art as photoconductive materials and many photoconductive elements have been prepared and used employing polyvinyl carbazole and related materials. As shown in U.S. Pat. No. 3.162,532 for example, polyvinylcarbazole is an ultraviolet photoconductor and its light response can be greatly enhanced and sensitized into the visible by complexing with an aromatic charge-transfer acceptor such as 2,4,7-trinitrofluorenone. This discovery has extended the scope of organic electrophotography and various articles have been published and patents issued in this field.
The prior art contains a large body of patents and articles concerned with the preparation and use of materials prepared from carbazoles. A main photoconductor of interest has been a polyvinylcarbazole polymer prepared fron N-vinylcarbazole which has been found to function in a useful manner in photoconductive layers. It is also known to form complexes of polyvinylcarbazoles with an electron acceptor such as 2,4,7-trinitro-9-fluorenone for use in photoconductive applications. A complex of this type is disclosed for example in U.S. Pat. No. 3,666,458.
In addition, the photosensitization of poly N-vinylcarbazoles has been studied as shown for example by Hoegl et al, "Photochemistry and Photobiology", 1972, Vol. 16, pps. 335-352, and in "Photographic Science and Engineering", Vol. 14, No. 3, May-June, 1970, pps. 205-209. Further, in an article by Schaffert in IBM J. of Research & Development, January, 1972, pps. 75-89, there is set forth a discussion of an organic photoconductor consisting essentially of the reaction product of 2,4,7-trinitro-fluorenone with poly-N-vinylcarbazole. Further, the photoelectric affects in polymers and their sensitization by doping materials was studied in "Journal of Physical Chemistry", by Hoegl, Vol. 69, No. 3, March 1965 at pps. 755-765.
The prior art has also been concerned with electrophotographic materials of this type formed by substitution by various substituents on aromatic portions of the carbazole molecule. For example U.S. Pat. No. 3,575,698 discloses brominated poly-N-vinylcarbazoles, and U.S. Pat. No. 3,848,237 discloses organic photoconductors comprising polymerized vinylcarbazoles which are reacted with trinitrofluorenone. In addition, in U.S. Pat. No. 3,627,524, chlorine-substituted poly-9-vinylcarbazole photoconductor polymers and copolymers were studied which were substituted in the 3-position by chloro or in the 3- and 6-position by chlorine atoms. A similar prior U.S. Pat. No. 3,705,031 discloses electrophotographic photosensitive materials which contain chlorine and bromine substituted 9-vinylcarbazole units. A similar disclosure including chloro- and iodo-substituents may be found in U.S. Pat. No. 3,702,764. In U.S. Pat. No. 3,421,891, polyvinylcarbazoles containing bromo substituents in the 3- and/or 6-position were studied as electrophotographic materials, and U.S. Pat. No. 3,595,648 discloses organic photoconductive materials comprising poly-N-vinyl-3-nitroso carbazoles together with copying paper prepared using this material.
In U.S. Pat. No. 3,485,624 there is disclosure of a photoconductive coating containing poly-N-vinylcarbazole obtained by heat treating the coatings at elevated temperatures to improve the photographic speed. Similarly, in the progression of the art, U.S. Pat. No. 3,684,506 discloses dimers of N-alkenylcarbazoles wherein the carbazole groups may also contain substituents such as alkyl, aryl, cycloalkyl, alkaryl, cyano, carboxylic acid or amino groups. Similarly, U.S. Pat. No. 3,526,502 discloses photoconductive materials comprising polymeric compounds such as poly-N-vinylcarbazole and brominated materials of this type, and U.S. Pat. No. 3,552,959 relates to electrophotographic copying paper using poly-N-vinylnitrocarbazole as an organic photoconductive material.
In U.S. Pat. No. 3,697,264, photoconductive carbazole polymers are described which have recurring polyvinylcarbazole units wherein the aromatic portion of the molecule may be substituted, particularly in the 3 -position, by a substituent such as alkyl, aryl, cycloalkyl, halogen or hydrogen. Also, a similar polyvinylcarbazole substituted in the 3-position by an azo radical is disclosed in U.S. Pat. No. 3,583,869. Further, in British Pat. No. 1,254,217, there are disclosed organic photoconductive materials and electrically conductive layers containing these materials which comprise a substituted polyvinylcarbazole wherein the substituent may be hydrogen, chlorine, bromine, iodine, or nitro.
In German Offenlegungsschrift Nos. 2,313,632 and 2,360,102 there are disclosed bis-carbazoles and complexes of these bis-carbazoles with electron acceptors such as 2,4,7-trinitro-9-fluorenone and 2,4,5,7-tetranitro-9-fluorenone.
U.S. Pat. No. 3,811,879 discloses photoconductive insulating coating compositions which include a crystallization prevention agent. In this system, there is used a charge-transfer agent comprising the complex of a poly-N-vinylcarbazole and trinitrofluorenone material and this charge-transfer complex is improved by increasing the trinitrofluorenone content. None of the polyvinylcarbazoles used have substituents on the organic portions of the molecule.
The present invention provides improved organic charge-transfer complexes which may be applied either as smooth polymer layers or as microcrystalline molecular compounds embedded in an inert binder. A charge-transfer complexes comprise the reversible formation of a larger chemical entity by the association of two molecules and visually exhibiting a visible light absorption, extra to the absorption of the separate components, which is the result of an intermolecular transfer of charge from one molecule, the donor, to the other, the acceptor.